1. Field of the Invention
The present invention relates to a specialized improvement in the apparatus used for fluid mixture monitoring, constituent analysis, and composition control, as disclosed and described in the referenced patent. More particularly, it relates to a monolithic or integral construction of an acoustic waveguide tube as disclosed in said patent and to a method and apparatus for undesired acoustic energy cancellation in waveguide tube assemblies.
The acoustic waveguide tube disclosed in the referenced patent is a sensor utilizing measurements derived from the velocities of sound in fluids, usually gases. It is comprised of an elongated chamber for containing a test fluid and having a means for generating, in response to an electrical impulse, sound waves at one end of the chamber. Receiving transducers at the other end of the chamber convert the sound waves into electrical impulses to permit measurement of time of flight of the sound waves. As a result, time of flight measurements of sound in different fluids can be obtained.
There exist many commercial uses for the invention disclosed in the referenced patent in which the fluids must be maintained at the highest possible levels of purity. Specifically, in electronic device manufacturing, "micro-contamination" or adulteration of process fluids with even extremely minute amounts of unwanted chemicals or particulate material has been identified as causing deleterious effects on resulting device quality and process yields. In other applications, it is important that process fluids be completely sealed within an instrument of integral construction to prevent any possible exposure to potential sources of combustion.
It has therefore been determined that it is advantageous if the acoustic waveguide tubes used in the patented instruments are of monolithic construction so as to eliminate the possibility of microcontamination or leakage of the contained fluids. In this context, monolithic construction is defined to mean that the containment chamber of the waveguide tube, which is the essential component of the instrument, be made an integral structure whereby it comprises a unitary element which cannot be disassembled into a subparts without invasive deconstruction.
2. Description of the Prior Art
In acoustic chambers used to contain fluids in which sound wave time of flight measurements are made, unwanted oscillations result from mechanical coupling between the sound wave sending and receiving elements, causing the latter to begin to oscillate during the time of flight of sound waves through the contained fluids. In other words, sound waves may travel through the containment chamber material to the receiving transducers faster than sound waves travel through the contained fluids.
Prior to the present invention, the best state of the art in construction of acoustic chambers included the use of "O" rings, threaded joints between dissimilar materials, and the use of visco-elastic materials in their construction for the purpose of reducing the amplitude of such unwanted oscillations in the receiving transducers. Examples of such construction are described in the referenced patent.
In related fields, active techniques have been disclosed in engineering literature for attenuating interfering acoustic energy by utilizing subsequent pulses of the sending transducers. Such techniques, utilized in such fields as medical imaging instrumentation or ambient sound cancellation, have been found to be impractical when applied to waveguide tubes of monolithic construction.
The present invention, a passive technique, vastly improves the state of the art by enabling the forming and functioning of acoustic waveguide chambers from a single piece of solid material, thus eliminating all possible diffusion paths between the outside environment and the fluids contained within. Acoustic waveguide tubes of the present invention can be fabricated of glass, ceramics, stainless steel, or other corrosion resistant material, with all parts fused or welded together and the transducing elements located externally so they do not contact the contained fluids.